Q-Chem is equipped with several standard ECP sets which are specified using the ECP keyword within the *$rem* block. The built-in ECPs, which are described in some detail at the end of this Chapter, fall into four families:

The Hay-Wadt (or Los Alamos) sets (fit-HWMB and fit-LANL2DZ)

The Stevens-Basch-Krauss-Jansien-Cundari set (fit-SBKJC)

The Christiansen-Ross-Ermler-Nash-Bursten sets (fit-CRENBS and fit-CRENBL)

The Stuttgart-Bonn sets (SRLC and SRSC)

Besides the ones above, a common “def2-ECP" needs to be used with Karlsruhe basis sets for elements Rb-Rn (see section 8.3).

References and information about the definition and characteristics of most of these sets can be found at the EMSL site of the Pacific Northwest National Laboratory:[EMS()]

Each of the built-in ECPs comes with a matching orbital basis set for the valence electrons. In general, it is advisable to use these together and, if you select a basis set other than the matching one, Q-Chem will print a warning message in the output file. If you omit the BASIS *$rem* keyword entirely, Q-Chem will automatically provide the matching one.

The following *$rem* variable controls which ECP is used:

ECP

Defines the effective core potential and associated basis set to be used

TYPE:

STRING

DEFAULT:

No ECP

OPTIONS:

General, Gen

User defined. (

$ecpkeyword required)Symbol

Use standard ECPs discussed above.

RECOMMENDATION:

ECPs are recommended for first row transition metals and heavier elements. Consul the reviews for more details.

If you wish, you can use different ECP sets for different elements in the system. This is especially useful if you would like to use a particular ECP but find that it is not available for all of the elements in your molecule. To combine different ECP sets, you set the ECP and BASIS keywords to “GEN” or (equivalently) “GENERAL”, and then add a *$ecp* block and a *$basis* block to your input file. In each of these blocks, you must name the ECP and the orbital basis set that you wish to use, separating each element by “****”. There is also a built-in combination that can be invoked specifying ECP = fit-LACVP. It assigns automatically 6-31G* or other suitable type basis sets for atoms H–Ar, while uses fit-LANL2DZ for heavier atoms.

**Example 9.203** Computing the HF/fit-LANL2DZ energy of AgCl at a bond length of 2.4 Å.

$molecule 0 1 Ag Cl Ag r r = 2.4 $end $rem METHOD hf Hartree-Fock calculation ECP fit-lanl2dz Using the Hay-Wadt ECP BASIS lanl2dz And the matching basis set $end

**Example 9.204** Computing the single point energy of HI with B3LYP/def2-SV(P) (using def2-ECP for I).

$molecule 0 1 H 0.0 0.0 0.0 I 0.0 0.0 1.5 $end $rem METHOD b3lyp BASIS def2-sv(p) ECP def2-ecp SYMMETRY false SYM_IGNORE true THRESH 14 SCF_CONVERGENCE 8 $end

**Example 9.205** Optimization of the structure of Se using HF/fit-LANL2DZ, followed by a single-point energy calculation at the MP2/fit-LANL2DZ level.

$molecule 0 1 x1 x2 x1 xx Se1 x1 sx x2 90. Se2 x1 sx x2 90. Se1 90. Se3 x1 sx x2 90. Se2 90. Se4 x1 sx x2 90. Se3 90. Se5 x2 sx x1 90. Se1 45. Se6 x2 sx x1 90. Se5 90. Se7 x2 sx x1 90. Se6 90. Se8 x2 sx x1 90. Se7 90. xx = 1.2 sx = 2.8 $end $rem JOBTYPE opt METHOD hf ECP fit-lanl2dz $end @@@ $molecule read $end $rem METHOD mp2 MP2 correlation energy ECP fit-lanl2dz Hay-Wadt ECP and basis SCF_GUESS read Read in the MOs $end

**Example 9.206** Computing the HF geometry of CdBr using the Stuttgart relativistic ECPs. The small-core ECP and basis are employed on the Cd atom and the large-core ECP and basis on the Br atoms.

$molecule 0 1 Cd Br1 Cd r Br2 Cd r Br1 180.0 r = 2.4 $end $rem JOBTYPE opt Geometry optimization METHOD hf Hartree-Fock theory ECP gen Combine ECPs BASIS gen Combine basis sets PURECART 1 Use pure d functions $end $ecp Cd srsc **** Br srlc **** $end $basis Cd srsc **** Br srlc **** $end